1. Field of the Invention
The present invention relates to an inkjet printhead, and more particularly, to an inkjet printhead for use in an image forming apparatus that forms images by ejecting droplets of ink from multiple nozzles onto a recording medium.
2. Discussion of the Background
Inkjet printing technologies are employed in many image forming apparatuses, such as printers, facsimiles, photocopiers, plotters, and multifunctional machines incorporating several of these capabilities. In general, an inkjet printer employs a fluid-ejecting device called a printhead that forms images by ejecting droplets of liquid ink from multiple nozzles onto recording media, such as paper, transparency film, etc., passing through a print zone.
Typically, an inkjet printhead contains an array of multiple nozzles in fluid communication with channels or chambers holding ink, and an actuator that pressurizes the ink chambers to expel ink in droplets from the corresponding nozzles. To date, inkjet printheads are manufactured with various configurations of nozzle arrays and/or actuators for various types of inkjet printers. For example, a movable printhead with a relatively short array of nozzles is employed in serial inkjet printers, which print images while moving back and forth along a scanning axis to traverse the width of the print zone. By contrast, a stationary printhead with an elongated nozzle array (in particular, one spanning the width of the print zone) is designed for line inkjet printers, which can perform printing without reciprocating movement along the scanning axis. Different types of printheads are constructed with different types of actuators, such as piezo-actuators formed of piezoelectric elements, thermal actuators using resistive heaters, electrostatic actuators that work by generating electrostatic forces, etc.
In most inkjet printers, a printhead is composed of multiple identical head modules each having chambers for holding ink, a driver or actuator for pressurizing the ink chambers, and a nozzle plate defining an array of nozzles, all manufactured with high precision and integrated into a single precision assembly. These head modules are mounted on a single mount base or carriage, with the nozzle plates forming a nozzle face in a particular arrangement according to the type (e.g., serial or line) of the printer into which the printhead is incorporated. Such modular design allows for repairing defective modules without requiring replacement of the entire printhead, and facilitates manufacture of a wide-array printhead for full-line inkjet printers that can perform printing at extremely high speed.
What is essential for good performance of such a modular printhead is the precision with which the multiple head modules are assembled into a single unit. This includes horizontal accuracy in positioning each head module with respect to one another in the horizontal plane, as well as vertical accuracy in positioning each head module on the mount base so that the printhead installed in a printer has its nozzle face at a consistent distance close to a recording medium passing throughout the print zone. For example, today's inkjet printers require a horizontal accuracy of within ±10 μm in terms of the amount of deviation from perfect alignment between nozzle arrays, and a consistent vertical gap of within 1 mm or smaller between the nozzle face and the recording medium for high definition inkjet printing.
Various construction techniques have been proposed to provide a modular printhead assembly with the required high horizontal and vertical positioning accuracies.
For example, one conventional technique provides a printhead constructed with multiple head modules, each having a nozzle plate and a substrate connected together, mounted on a single carriage having multiple sets of standard level surfaces (hereinafter “datum surfaces”) defined therein. Each head module has a positioning member defined in the substrate, and is positioned along x-, y-, and z-axes in the carriage by contacting the positioning member with the corresponding datum surfaces.
This method is designed to arrange the multiple head modules in line on the single carriage, but fails to ensure precise alignment of the nozzle arrays and good positioning of the nozzle face. That is, providing the positioning member on the substrate but not on the nozzle plate cannot compensate for variations in the connection between the substrate and the nozzle plate, resulting in misalignment of the nozzle arrays along the horizontal x- and y-axes. Further, dimensional variations inherent both in the positioning members and the datum surfaces affect positioning of the nozzle plates along the vertical axis, resulting in an inconsistent gap between the nozzle face and the recording medium.
Another conventional technique provides a printhead assembly having multiple replaceable head modules staggered on a mount base extending parallel to the width of a print zone, in which each head module has a set of positioning holes defined in the nozzle plate for engagement with a set of positioning pins disposed in the mount base. Each nozzle plate is positioned in a horizontal plane by engaging the positioning holes with the positioning pins, and in a vertical direction with a screw-fixed cover plate covering the surface of the mount base except for the nozzle arrays while securing edges of the nozzle plates against the mount base.
This method provides proper positioning of the nozzle plates in the horizontal plane by engaging the positioning pins and holes, effecting good alignment between the multiple nozzle arrays. However, using the cover plate for securing the nozzle plates in place results in certain drawbacks. Firstly, interposing the cover plate, which has a sufficient thickness to withstand mechanical stress, between the nozzle face and the recording medium increases the distance between the nozzle face and the recording medium in the print zone. Moreover, securing the multiple nozzle plates with the single cover plate results in poor maintainability of the printhead since replacement of even a single defective module requires demounting of the entire printhead unit for removing the screw-fixed cover plate.
Still another conventional technique proposes a printhead assembly with multiple head modules precisely positioned in a carriage using image data processing. According to this method, each head module has a nozzle plate with a set of alignment marks defined thereon, and a frame with an adjustment lever projecting therefrom for adjusting the position of the head module in the carriage. The assembly process includes vertically positioning the head module by engaging the adjustment lever with a holder disposed on the carriage, and adjusting the horizontal position of the nozzle plate to match a reference plane based on the position of the alignment marks detected and processed by imaging equipment.
This method enables precise alignment of multiple nozzle arrays in the horizontal plane using image data processing, but is insufficient where the vertical positioning is affected by an accumulation of variations in engaging the adjustment lever and the carriage holder. Also, this method has a drawback in that positioning the head modules using imaging equipment makes it impossible or impractical for a user to replace a defective nozzle module in the printhead assembly once it is installed.
Hence, what is required is a printhead assembly with a simple but high-precision positioning mechanism for use in an inkjet printer, which can properly position multiple head modules and nozzle arrays not only in a horizontal plane but also vertically to produce a consistent narrow gap between the nozzle face and the recording medium.